1. Field of the Invention
The present invention relates to a system for broadband optical amplification, particularly to a dual window Raman amplifier, and more particularly to a dual window, bi-directional Raman amplifier.
2. Description of the Prior Art
Currently, dense wavelength division multiplexing (DWDM) technology is being used to dramatically improve high capacity optical transmission systems. Systems are now being developed that will allow transmission of 64 (and beyond) WDM channels on a single fiber. This represents a dramatic increase from existing 16 channel systems.
Broadband optical amplifiers will be essential components in such systems and will need to provide high enough bandwidth, low enough noise, and sufficient power to minimize channel crosstalk and degradation due to fiber nonlinearities. In addition, it is important that the amplifiers provide a graceful upgrade path for the customer. It is very likely that customers initially will only require a fraction of the maximum system capacity, while maintaining the option to expand the capacity at a later point. More specifically, they will require amplifiers that can be upgraded (as opposed to being replaced) to the full system capacity at a small additional cost.
Recently, it has been demonstrated that the bandwidth of erbium doped fiber amplifiers (operating around 1550 nm) can be expanded to more than 80 nm using a split band technique. Such amplifiers would easily accommodate 100 wavelength channels at 100 GHz spacing. However, current systems where the wavelength channels occupy a large contiguous bandwidth have the significant disadvantage that they suffer substantial penalties due to stimulated Raman scattering. These penalties occur because the lower wavelength channels loose power (and thereby signal to noise ratio) as they pump the upper wavelength channels. Also, an erbium doped fiber amplifier with the full 80 nm bandwidth may not necessarily provide the customer with the desired upgrade path.